In aeronautical terminology, an apparatus for the presentation of flight data superimposed on the pilot's outside view by semi-transparent imaging means is known as a "head-up display" (HUD). Such a display is particularly useful during landing and during the release of weapons or cargo from the aircraft (i.e., flight and aiming operations).
The data required for the above mentioned flight and aiming operations is prepared or processed by an on-board computer which usually applies digital information to the face of a cathode ray tube (CRT) where the data appears in the form of symbols and images. By means of an optical system, the images thus generated by the CRT are projected to infinity and reflected into the pilot's field of view by a semi-transparent window or combiner to give the pilot a simultaneous view of the image itself and of the distant scene (i.e., the approaching landing field).
Elementary head-up displays are described in U.S. Pat. Nos. 3,816,005 and 3,851,303 both of which are assigned to the Assignee of the present invention. Some head-up displays (i.e., U.S. Pat. No. 2,887,927; 3,438,600; etc.) have many mechanical components. Others have very complicated electronic processing circuits to produce the desired CRT images. All systems suffer from the possibility of alignment errors and optical-mechanical or electronic failure.
The art has recognized the problem of alignment of a head-up display (i.e., U.S. Pat. No. 4,108,025). Fairly detailed, and relatively complicated, optical alignment test systems have been developed (i.e., U.S. Pat. No. 4,400,731). The '731 patent, for example, discloses an apparatus for checking the alignment of a head-up display on the ground or at a test stand physically removed from the normal location of the head-up display. However, such an approach is basically inadequate when the pilot's requirements are considered. Basically, a HUD can serve its intended purpose only if the pilot has complete confidence in it. The accuracy and correct display of the symbology, along with the display availability, are crucial factors in gaining pilot confidence. An on-board system is needed.
Because the electronics associated with a HUD transforms digital data into visual images, it should be evident that an on-board HUD, employing redundant channel comparison on a complete basis (i.e., from input to final image output) is not possible. First of all, there is no simple way to compare visual images to visual images. More importantly, space and other physical constraints make such a system impractical. For this reason, in a so called "redundant channel system", the CRT, the relay optics, and the optical combiner are shared and only the signal processing portion is redundant. Thus, a significant portion of the entire HUD signal processing train, in terms of failure probability, is left unprotected. Therefore, dual redundant signal processing channels that are cross-compared at the output can only do a partial job. More importantly, hardware is doubled and the effect is a degraded mean time between failures (MTBF).
Because of the importance of a head-up display to flight safety and because of the importance of such a display in military aircraft in insuring that the mission of that aircraft may be completed, it is important to have an aircraft head-up display by which the pilot, or other user, can independently check the alignment of that display and the integrity of the signal processing train. A simple system which will allow such a check to be performed fairly quickly would be especially appreciated by the art and would also contribute to improved flight safety and combat mission effectiveness.